Kaap 309: Muscular System III (Contraction, Advanced Terminology, Reflexes)

Muscle Twitch

• response of a muscle to a single stimulation

• Three periods:

1. Latent: Delay → stimulus happens, but no contraction yet.

2. Contraction: Muscle fibers shorten and tension rises

3. Relaxation: Muscle relaxes, Ca²⁺ goes back to SR.

Muscle Fatigue

Decreased ability of a muscle to contract despite stimulation (Definition: Muscle cannot contract fully even with stimulation.)

High-Intensity Exercise Causes

• K⁺ accumulation in T-tubules

• ADP and Pi buildup → interferes with cross-bridge cycling

Low-Intensity (Long-Duration) Causes

• Fuel depletion (low glycogen/glucose)

• Electrolyte loss

• Central fatigue (reduced neural drive)

Types of contractions

• Isometric:

  • Muscle tenses but does not change length (no movement).

  • Example: Holding a plank.

• Isotonic:

  • Muscle changes length while tension remains constant.

  • Concentric: Muscle shortens (lifting a weight).

  • Eccentric: Muscle lengthens (lowering a weight)

Type	Description	Example
Isotonic (concentric)	Muscle shortens while generating force	Lifting a weight
Isotonic (eccentric)	Muscle lengthens while resisting	Lowering a weight slowly
Isometric	Muscle contracts but doesn’t shorten	Holding a plank or wall sit

Fascicle arrangement

• Parallel

  • Fibers run parallel to the long axis of the muscle.

  • Can shorten a lot → good range of motion.

  • Example: Sartorius

• Fusiform

  • Spindle-shaped parallel fibers with a bulge in the middle.

  • Moderate force, good for speed.

  • Example: Biceps brachii

• Circular

  • Fibers arranged in a circle → control openings.

  • Example: Orbicularis oris (around the mouth)

• Pennate (feather-like)

  • Fibers attach to tendon at an angle → generate more force, less range.

  • Unipennate: fibers on one side of tendon → Example: Extensor digitorum

  • Bipennate: fibers on both sides → Example: Rectus femoris

  • Multipennate: tendon branches within muscle → Example: Deltoid

Attachments: origin and insertion

• Origin (red):

– “stable” segment of a muscle

– Typically proximal

• Insertion (blue):

– “moveable” segment of a muscle

– Typically distal

Tip: “Origin = Anchor, Insertion = Moves”

Agonist and Antagonist

• Agonist

– Prime mover of a joint during contraction

• Antagonist

– Opposes the prime mover

• Agonist (Prime Mover):
  The muscle that does the main work to produce a movement.

  • Example: When you bend your elbow, the biceps is the agonist because it contracts to lift the forearm.

• Antagonist:
  The muscle that opposes the movement of the agonist.

  • Example: When bending your elbow, the triceps is the antagonist because it stretches and resists the movement.

Synergists & Fixator

• Synergist

– Assists the prime mover

• Fixator

– Prevents movement, "fixates" a bone, holds bone steady

Lever Systems

Class	Arrangement (F = Fulcrum, E = Effort, L = Load)	Example	Notes

1st

F between E & L

Neck (nodding)

Mechanical advantage or disadvantage

2nd

L between F & E

Standing on tiptoes

Always mechanical advantage

3rd

E between F & L

Biceps flexion

Most common in body

• 1st class: Fulcrum in middle → neck nodding

• 2nd class: Load in middle → tiptoe → always mechanical advantage

• 3rd class: Effort in middle → biceps → most common

First class levers

Second class levers

Third class levers

Reflex Arcs

• Types:

  • Inborn (intrinsic): Unlearned, involuntary (knee-jerk)

  • Learned (acquired): Practice → automatic (riding bike)

• Properties: Quick, involuntary, stereotyped, requires stimulation

Reflex Properties

1. Requires stimulation → happens because something triggers it.

2. Quick → happens fast, before you can think.

3. Involuntary → you don’t choose to do it.

4. Stereotyped → same response every time.

Mnemonic: QISS → Quick, Involuntary, Stereotyped, Stimulated

Muscle Spindles

• modified muscle fibers with the primary function of detecting and signaling muscle length (body movement)

• Typically found near the musculotendinous junction

• Proprioceptors

Intrafusal and Extrafusal

• Intrafusal fibers

– Within (inside) the muscle spindle

– Gamma motor neurons

– Sensory (afferent) fibers

 Primary & Secondary

• Extrafusal fibers

– "regular" muscle fibers

– Alpha motor neurons

• Intra = inside spindle → senses stretch

• Extra = outside spindle → does the work (contracts the muscle)

Stretch Reflex

• help the nerves system smoothly coordinate skeletal mm.

• What do we need to know?

– Length of the muscle (muscle spindles)

 How much (degree)

 How fast (rate)

[Golgi] Tendon Organs

• small, loose collagen fibers innervated by 1+ nerve fibers and detect tension

• Located in the tendons (near the musculotendinous junction)

• Proprioceptors

Tendon Reflex

• help to prevent muscles and tendons from tearing during stretching force

• What does it need to know?

  • Tension of the muscle (tendon organs)

• Polysynaptic reflex (results in muscle relaxation)

1. Afferent impulse to the cerebellum

2. Contracting muscle is inhibited and antagonist muscle is activated

Take Home Points

• There are three classes of levers found in the human body

• All reflex arcs must contain 5 essential components and 4 properties

• Stretch and tendon reflexes are the most common somatic reflexes

• Stretch Reflex: Protects muscle length → monosynaptic

• Tendon Reflex: Protects tendons → polysynaptic, relaxes contracting muscle

Tip: Stretch = “length check”, Tendon = “tension check”